Harnessing the properties of wave-based excitations requires a full control of their symmetry. A great degree of manipulation can be induced by simply acting on the geometry of the structures interacting with the waves: such strategy has led to impressive results in the field of photonics, where particular optical elements, photonic crystals and metamaterials have been extensively used to achieve milestone results in fundamental physics and applications. While a similar trend has been developed in acoustics, the manipulation of high-frequency waves, oscillating at GHz and beyond, has mostly eluded the research efforts, which concentrated in the kHz to MHz range and in-fluids wave propagation.
Here we show two experiments illustrating the manipulation of GHz waves on a solid-state, chip platform, making them interesting both as fundamental studies and applications as hybrid, on-chip systems combining acoustics with photonic and electronics nanodevices. The used monolithic GaAs platform favours the generation of surface acoustic waves and includes the possibility of fabricating metasurfaces in suspended material slabs: here we investigated GHz acoustic wave refraction at the interface between hole-patterned and unpatterned regions. Characterizing both with optical interferometry [1] and acoustic atomic force microscopy [2], we show how the metasurface symmetry influences wave refraction, demonstrating both ordinary, negative and asymmetric refraction, the latter not holding any link of the refracted angle upon mirroring of the incident one [1]. The different regimes can be accessed at different ranges of incident angles, facilitating the inclusion of such a scheme in GaAs polarization of light modulators/polarimeters via optomechanical interaction, already demonstrated at MHz frequency [3] and with promising preliminary results in the GHz range [4].
Additional symmetries can be accessed by enabling mechanical orbital angular momentum (OAM). To this end, we designed and experimentally characterized bulk acoustic wave resonators (BAWR) shaped as an Archimedean spiral for the generation of acoustic vortex beams propagating into a sapphire substrate. With a mechanical frequency around 4 GHz, tunable topological charge, and the capability to impart different OAM states to reflected light, our solid-state platform explores OAM-optomechanics, new opportunities for light-based communication and GHz topological acoustic effects [5].
[1] SZ, G. Biasiol, PVS, AP, Nat. Comm. 13, 5939 (2022)
[2] AP, M. Yuan, SZ, PVS, Phys. Rev. Appl. 20, 054054 (2023)
[3] SZ et al., Adv. Opt. Mat. 8, 1901507 (2020)
[4] AP et al., Adv. Opt. Mat. 12, 2401283 (2024)
[5] AP et al., arXiv:2410.17877 (2024)